Distribution and stabilization of fluid flow for interlayer chip cooling

ABSTRACT

A method of forming metallic pillars between a fluid inlet and outlet for two-phase fluid cooling. The method may include; forming an arrangement of metallic pillars between two structures, the metallic pillars are electrically connected to metallic connecting lines that run through each of the two structures, the arrangement of metallic pillars located between a fluid inlet and a fluid channel, the fluid channel having channel walls running between arrangements of the metallic pillars and a fluid outlet, whereby a fluid passes through the arrangement of metallic pillars to flow into the fluid channel.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with United States Government support underDARPA Agreement No. FA8650-14-c-7466. THE GOVERNMENT HAS CERTAIN RIGHTSIN THIS INVENTION.

BACKGROUND

The present invention generally relates to two-phase cooling forintegrated circuits (ICs), and more particularly to the formation ofmicro-metallic pillars for IC cooling, flow stabilization, anddistribution.

A two-phase liquid cooling system could efficiently suppress junctiontemperatures with less power consumption using vaporization near highperformance integrated circuits (ICs). Specifically, for 3D ICs,micro-channels shall be embedded inside stackable silicon dies to removeheat and obtain certain temperature profile.

SUMMARY

According to one embodiment of the present invention, a method offorming metallic pillars between a fluid inlet and outlet for two-phasefluid cooling is provided. The method may include providing a firstsemiconductor structure having first metallic attachments on a topsurface of a first substrate and second metallic attachments on a bottomsurface of the first substrate, the first metallic attachments connectedto first connecting lines and the second metallic attachments connectedto second connecting lines, wherein the first connecting lines and thesecond connecting lines are in the first substrate; forming firstmetallic pillars on the second metallic attachments; providing a secondsemiconductor structure having third metallic attachments on a topsurface of a second substrate and fourth metallic attachments on abottom surface of the second substrate, the third metallic attachmentsconnected to third connecting lines and the fourth metallic attachmentsconnected to fourth connecting lines, wherein the third connecting linesand the fourth connecting lines are in the second substrate; formingsecond metallic pillars on the fourth metallic attachments; forming anassembled semiconductor structure by bonding the first metallic pillarsto the second metallic pillars using a conductive material, wherein afluid channel separates the bottom surface of the first semiconductorstructure from the bottom surface of the second semiconductor structure;and enclosing the assembled structure within an enclosure, wherein afluid can enter the enclosure through an inlet port, pass between thebonded metallic pillars and through the fluid channel, and exit theenclosure through an outlet port.

According to another embodiment of the present invention, a structurefor two-phase fluid cooling of integrated circuits (IC's) is provided.The structure may include an enclosure; a first semiconductor structure;a second semiconductor structure located above the first semiconductorstructure, wherein the first semiconductor structure and the secondsemiconductor structure are within the enclosure; and an arrangement ofmetallic pillars located between the first semiconductor structure andthe second semiconductor structure, wherein the arrangement of metallicpillars electrically and thermally connect the first semiconductorstructure to the second semiconductor structure, the arrangement ofmetallic pillars located in a fluid channel separating the firstsemiconductor structure and the second semiconductor structure; whereina fluid enters the enclosure at a fluid inlet and passes between thefirst semiconductor structure and the second semiconductor structurethrough the fluid channel and exits the enclosure at a fluid outlet, thefluid cooling the arrangement of metallic pillars through a two-phasecooling process.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description, given by way of example and notintended to limit the invention solely thereto, will best be appreciatedin conjunction with the accompanying drawings, in which:

FIG. 1 is a cross sectional side view of a semiconductor structure isprovided, according to an embodiment;

FIG. 2 is a cross sectional side view of the semiconductor structure andillustrates the bonding of a glass handler to the semiconductorstructure, according to an embodiment;

FIG. 3 is a cross sectional side view of the semiconductor structure andillustrates the thinning of a backside of the semiconductor structure,according to an embodiment;

FIG. 4 is a cross sectional side view of the semiconductor structure andillustrates the formation of a metal layer on the backside of thesemiconductor structure, according to an embodiment;

FIG. 5 is a cross sectional side view of the semiconductor structure andillustrates the formation of first metallic pillars on the semiconductorstructure, according to an embodiment;

FIG. 6 is a cross sectional side view of an assembled semiconductorstructure and illustrates the formation of a second structure bonded tothe first structure, according to an embodiment;

FIG. 7 is a cross sectional side view of the assembled semiconductorstructure and illustrates a fluid path of flow through the assembledsemiconductor structure, according to an embodiment;

FIG. 8 is a cross sectional top view of fluid channels and illustratesthe fluid path of flow through parallel fluid channels, according to anembodiment;

FIG. 9 is a cross sectional top view of fluid channels and illustratesthe fluid path of flow through parallel fluid channels, according to anembodiment;

FIG. 10 is a cross sectional top view of fluid channels and illustratesthe fluid path of flow through parallel fluid channels, according to anembodiment;

FIG. 11 is a cross sectional top view of fluid channels and illustratesthe fluid path of flow through radial fluid channels, according to anembodiment;

FIG. 12 is a cross sectional top view of fluid channels and illustratesthe fluid path of flow through radial fluid channels, according to anembodiment; and

FIG. 13 is a cross sectional top view of fluid channels and illustratesthe fluid path of flow through radial fluid channels, according to anembodiment.

The drawings are not necessarily to scale. The drawings are merelyschematic representations, not intended to portray specific parametersof the invention. The drawings are intended to depict only typicalembodiments of the invention. In the drawings, like numbering representslike elements.

DETAILED DESCRIPTION

Detailed embodiments of the claimed structures and methods are disclosedherein; however, it can be understood that the disclosed embodiments aremerely illustrative of the claimed structures and methods that may beembodied in various forms. This invention may, however, be embodied inmany different forms and should not be construed as limited to theexemplary embodiments set forth herein. Rather, these exemplaryembodiments are provided so that this disclosure will be thorough andcomplete and will fully convey the scope of this invention to thoseskilled in the art. In the description, details of well-known featuresand techniques may be omitted to avoid unnecessarily obscuring thepresented embodiments.

References in the specification to “one embodiment”, “an embodiment”,“an example embodiment”, etc., indicate that the embodiment describedmay include a particular feature, structure, or characteristic, butevery embodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

For purposes of the description hereinafter, the terms “upper”, “lower”,“right”, “left”, “vertical”, “horizontal”, “top”, “bottom”, andderivatives thereof shall relate to the disclosed structures andmethods, as oriented in the drawing figures. The terms “overlying”,“atop”, “on top”, “positioned on” or “positioned atop” mean that a firstelement, such as a first structure, is present on a second element, suchas a second structure, wherein intervening elements, such as aninterface structure may be present between the first element and thesecond element. The term “direct contact” means that a first element,such as a first structure, and a second element, such as a secondstructure, are connected without any intermediary conducting, insulatingor semiconductor layers at the interface of the two elements.

In the interest of not obscuring the presentation of embodiments of thepresent invention, in the following detailed description, someprocessing steps or operations that are known in the art may have beencombined together for presentation and for illustration purposes and insome instances may have not been described in detail. In otherinstances, some processing steps or operations that are known in the artmay not be described at all. It should be understood that the followingdescription is rather focused on the distinctive features or elements ofvarious embodiments of the present invention.

The present invention generally relates to two-phase cooling forintegrated circuits (ICs), and more particularly to the formation ofmicro-metallic pillars for IC cooling, flow stabilization, anddistribution. The present invention includes a fabrication, designmethod and arrangements of micro-metallic pillars in 3D chip stacks. Thepillar arrangements facilitate, among other things, both controllingtwo-phase flow and distributing flow and bubble generation inmicro-channels. One or more arrangements of metallic pillars cangenerate uniform flow velocity fields and enhance two-phase flowstability. The metallic pillars can provide electrical and thermalinterconnections between semiconductor structures (e.g., chips) as wellas provide structural support of chips in a 3D stack. Exemplaryembodiments by which to form micro-channels and metallic pillars aredescribed in detail below referring to the accompanying drawings FIGS.1-13.

With reference to FIG. 1, a demonstrative illustration of a firststructure 100 during an intermediate step of a method of fabricatingmicro-metallic pillars is provided, according to an embodiment. Morespecifically, the first structure 100 can include a first metallicattachment 106 on a substrate 102.

The substrate 102 may include; a bulk semiconductor substrate, a layeredsemiconductor substrate (e.g., Si/SiGe), a silicon-on-insulatorsubstrate (SOI), or a SiGe-on-insulator substrate (SGOI). The substrate102 may include any semiconductor material known in the art, such as,for example; Si, Ge, SiGe, SiC, SiGeC, Ga, GaAs, InAs, InP, or otherelemental or compound semiconductors. The substrate 102 may include, forexample; an n-type, p-type, or undoped semiconductor material and mayhave a monocrystalline, polycrystalline, or amorphous structure. In anembodiment, the substrate 102 is a bulk silicon substrate.

A first connecting line 104 a and a second connecting line 104 b may beformed in the substrate 102 using any line formation technique known inthe art, such as, for example, a trench and fill process. The first andsecond connecting lines 104 a, 104 b may be any conductive materialknown in the art, such as, for example, tungsten (W). The firstconnecting line 104 a may be exposed on a top surface of the substrate102 and the second connecting line 104 b can be buried in the substrate102.

The first metallic attachment 106 may be formed on a top surface of thesubstrate 102 and may be in contact with the first connecting line 104a. The first metallic attachment 106 can be formed using any depositiontechnique known in the art, such as, for example, epitaxial growth,chemical vapor deposition (CVD), physical vapor deposition (PVD), oratomic layer deposition (ALD). The first metallic attachments may bedeposited as an attachment layer and patterned to form the firstmetallic attachment 106 using any patterning technique known in the art,such as, for example, a mask and etching process. The first metallicattachment 106 can be any metallic material known in the art, such as,for example, a Sn—Pb alloy and/or BGA solder balls.

With reference to FIG. 2, a demonstrative illustration of the firststructure 100 during an intermediate step of a method of fabricatingmicro-metallic pillars is provided, according to an embodiment. Morespecifically, a handler substrate can be bonded to the top surface ofthe substrate 102 and the first metallic attachment 106 (flipped in FIG.2 for a subsequent thinning step described further with reference toFIG. 3) using any carrier bonding technique known in the art. Thehandler substrate can be bonded to the top surface of the substrate 102using a first adhesive layer 108 and a handler layer 110 as is known inthe art.

With reference to FIGS. 3 and 4, a demonstrative illustration of thefirst structure 100 during an intermediate step of a method offabricating micro-metallic pillars is provided, according to anembodiment. More specifically, a metal layer 111 may be formed on apolished back side of the substrate.

The backside of the substrate 102 may be polished (i.e., thinned) usingany polishing technique known in the art, such as, for example, agrinding and polishing process. This process may be performed usingconventional grinding, polishing and/or chemical etching means. Forinstance, a backside grind and polish can be performed on the backsurface of the substrate 102 to remove the material within 10-20 micronsof the bottom connecting line 104 b. A wet etch can then be performed toexpose the bottom connecting line 104 b on a thinned bottom surface ofthe substrate 102.

The metal layer 111 may be formed using any deposition technique knownin the art, such as, for example, epitaxial growth, chemical vapordeposition (CVD), physical vapor deposition (PVD), or atomic layerdeposition (ALD). The metal layer 111 may be formed on the thinnedbottom surface of the substrate 102 and on the exposed bottom connectingline 104 b. The metal layer 111 can be any metallic material known inthe art, such as, for example, Ti—Cu alloy.

With reference to FIG. 5, a demonstrative illustration of the firststructure 100 during an intermediate step of a method of fabricatingmicro-metallic pillars is provided, according to an embodiment. Morespecifically, a metallic pillar 122 may be formed on a second metallicattachment 112.

The second metallic attachment 112 may be formed by patterning the metallayer 111 using any patterning technique known in the art, such as, forexample, a mask and etching process. The metallic pillar 122 can beformed on the second metallic attachment 112 using any depositiontechnique known in the art, such as, for example, epitaxial growth,chemical vapor deposition (CVD), physical vapor deposition (PVD), oratomic layer deposition (ALD). A mask used to form the second metallicattachment 112 may also be used to form the metallic pillar 122. Themetallic pillar 122 can have a diameter (d) of about 25 μm and a height(h) of about 75 μm, however, other dimensions may be used. The pitch andlocation of the metallic pillar 122 may be adjustable to control atwo-phase flow in a cooling system's micro-channels. A solder 124 may beformed on the metallic pillar 122 using any known solder depositiontechnique known in the art. The solder 124 may be used to bond thestructure 100 to a second structure in subsequent steps described below.

With reference to FIG. 6, a demonstrative illustration of the firststructure 100 during an intermediate step of a method of fabricatingmicro-metallic pillars is provided, according to an embodiment. Morespecifically, a second structure 200 may be bonded to the firststructure 100.

The second structure 200 has a third metallic attachment 206 on a topsurface of a second substrate 202 and a fourth metallic attachment 212on a bottom surface of the second substrate 202. The third metallicattachment 202 and the fourth metallic attachment 212 are connected to athird connecting line 204 a and a fourth connecting line 204 b,respectively. It should be noted, the second structure 200 may have thesame elements as the first structure 100, such that like referencenumbers represent like elements. The second structure 200 may be alignedwith the first structure using a second handler substrate. The secondhandler substrate can be bonded to the top surface of the secondsubstrate 202 using a second adhesive layer 208 and a second handlerlayer 210 as is known in the art.

The assembling of the first structure 100 and the second structure 200may be done using any bonding process known in the art, such as, forexample, positioning the two structures to align metallic pillars andprovide physical contact between the solder 124 on the metallic pillar122 with solder on a second metallic pillar 222. The bonding process mayinclude holding the structures in alignment while heating the solder,and continuing to hold the alignment while allowing the entire assemblyto cool down and forming reflowed solder 125 between the metallic pillar122 and the second metallic pillar 222. It should be noted, the metallicpillar 122, reflowed solder 125 and a second metallic pillar 222 mayalso be referred to as a metallic pillar 123 for exemplary purposes. Themetallic pillar 123 may have a total height (th) of about 150 μm, butother heights may be used. In an embodiment, a fluid (e.g., coolant) mayflow between structure 100 and second structure 200 throughmicro-channels to cool the structures and other chip assemblies. Thefluid can pass between the metallic pillars 123 which can transfer heatfrom the semiconductor structures to the fluid. The fluid flow andpillar design are discussed further below. In an alternative embodiment,the depicted metallic pillars may also be formed from a non-metallicthermally conductive material. For example, pillars 122 and 222 may beconstructed from a non-metallic but thermally conductive material, whilea thermally conductive adhesive layer (not shown) may be used to bondthese pillars instead of the reflowed solder 125.

With reference to FIG. 7, a demonstrative illustration of an assembledstructure 250 is provided, according to an embodiment. Morespecifically, the assembled structure 250 illustrates a path of flow ofthe fluid as the semiconductor structures are cooled by transferringheat from the metallic pillars 123 (two-phase cooling process) to thefluid.

The fluid may have a path of flow passing through a reservoir 194, afilter 196, a pump 198, and the assembled structure 250. The assembledstructure 250 can include the first and second structures 100, 200within an enclosure 192 and on a base 190. The base 190 can be alaminate, such as, for example, an organic build-up substrate or aceramic single or multi-chip module. The enclosure 192 can include oneor more fluid inlet ports and outlet ports.

As the fluid enters the assembled structure 250 at the inlet port, thefluid passes between the metallic pillars 123 and the fluid channelbefore exiting the assembled structure 250 at the outlet port. As thefluid passes through the metallic pillars 123 (thermally cooling themetallic pillars 123 and the first and second structures 100, 200), thefluid may create bubbles 195.

It should be noted, this is an illustration of a parallel arrangementwhere the fluid enters from one side of the assembled structure 250 andexits on another side, however, a radial design may also be used wherethe fluid enters in a middle region of the assembled structure 250 andflows radial outward towards the sides of the assembled structure 250.The following drawings illustrate a few embodiments of a fluid path offlow as it passes through the metallic pillars 123 and a few alternativearrangements of fluid channels.

With reference to FIGS. 8-10, a demonstrative illustration of a fluidpassing through channels 301 is provided, according to an embodiment.More specifically, bubble generation/nucleation is illustrated relativeto alternative design embodiments of the metallic pillars 123. It shouldbe noted, FIGS. 8-13 are top views of the assembled structure 250illustrated in FIG. 7 taken along cross section line A-A.

Typically, rapid bubble growth in two-phase cooling system leads tosevere pressure drop fluctuation and a reverse flow of liquid and vapor.Before the fluid flow enters the micro-channel, a vapor bubble is notflavored because the vapor bubble may be larger than the hydraulicdiameter of the micro-channel inlet and may thus transiently orpermanently prevent liquid flow from entering the micro-channel.Utilizing the metallic pillars, vapor bubble generation can be triggeredand controlled inside micro-channels. Such flow instabilities are notfavored by a two-phase cooling system. Moreover, vapor quality in eachmicro-channel could be different as a result of non-uniform heatingprofiles in ICs. Thus, inlet fluid flow rate should be controlled tomatch the heating profile and prevent a high-vapor-quality area fromdrying out. Depending on design requirements, the following embodimentsare a few alternative arrangements for the metallic pillars 123.

A structure 300 may include a plurality of parallel channel walls 303.The fluid may flow from the inlet port, pass between the metallicpillars 123 and through the channels 301 during the two-phase process.As the fluid passes between the metallic pillars 123, bubbles 305 mayform and may be controlled by the arrangement of the metallic pillars123.

FIG. 8 depicts an embodiment of a two-phase flow design having parallelchannels 301 with a non-uniform arrangement of the metallic pillars 123and having an opening at the middle of the arrangement (i.e., anorifice). The pitch at the orifice may be larger than 50 μm and smallerthan a width of channel inlet, however, other dimensions may be used.The inlet flow rate may be passively controlled by the pitch at theorifice. Each channel 301 can share the same pressure head, where alarger pitch may allow for a relatively high flow rate for relativelyhigh heat loading, for example, core or hot spot area, while a smallpitch may generate relatively low flow rate for relatively low heatloading, for example, peripheral interface circuits and connectors. Theorifice stops large and rapid bubble from reversely flowing into theinlet area. After passing the Vena Contracta area, the fluid velocitycan reach maximum values. The increase in velocity comes at the expenseof fluid pressure, which leads to low local pressure in the VenaContracta area. If the local pressure is less than the vapor pressure ofthe liquid coolant, vapor bubbles may be generated inside the channels301. This feature helps the system in reducing boiling wall superheat,which is a case when the channel wall 303 surface temperatures arehigher than the liquid saturated temperature but phase change fromliquid to vapor may not be initiated.

FIG. 9 depicts an embodiment of a two-phase flow design having parallelchannels 301 with a staggered arrangement for the metallic pillars 123.The pitch may be larger than 25 μm and smaller than the width of channelinlet. The inlet flow rate may be passively controlled by thepermeability of pillar array. The permeability is controlled by pitchand the number of rows of metallic pillars 123. In the illustratedembodiment, two rows of metallic pillars 123 are used, however, anynumber of rows may be used. Since this arrangement results small andmultiple Vena Contracta areas, it may lead to small bubble generationsinside the channels 301. This feature may generate more uniform flowvelocity field and enhance two-phase flow stability.

FIG. 10 depicts an embodiment of a two-phase flow design having parallelchannels 301 with an inline arrangement of the metallic pillars 123.This embodiment is similar to the embodiment illustrated in FIG. 9, butcan allow for a higher flow rate using the same pitch and the samenumber of rows.

With reference to FIGS. 11-13, a demonstrative illustration of a radialstructure 400 is provided having a fluid passing through channels 401,according to an embodiment. More specifically, bubbles 405 areillustrated during bubble generation/nucleation relative to alternativedesign embodiments of channel walls 403 and the metallic pillars 123.

FIG. 11 depicts an embodiment of a two-phase flow design having radialchannels 401 with a non-uniform arrangement of the metallic pillars 123.The design and arrangement of the metallic pillars 123 may be similar tothe embodiment illustrated in FIG. 8 (i.e., including an opening at themiddle of the metallic pillars 123 arrangement).

FIG. 12 depicts an embodiment of a two-phase flow design having radialchannels 401 with a staggered arrangement for the metallic pillars 123.The design and arrangement of the metallic pillars 123 may be similar tothe embodiment illustrated in FIG. 9 (i.e., this embodiment may resultin small and multiple Vena Contracta areas, it may lead to small bubblegenerations inside the radial channels 401).

FIG. 13 depicts an embodiment of a two-phase flow design having radialchannels 401 with an inline arrangement of the metallic pillars 123. Thedesign and arrangement of the metallic pillars 123 may be similar to theembodiment illustrated in FIG. 10 (i.e., this embodiment can allow for ahigher flow rate using the same pitch and the same number of rows).

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the invention.The terminology used herein was chosen to best explain the principles ofthe embodiment, the practical application or technical improvement overtechnologies found in the marketplace, or to enable others of ordinaryskill in the art to understand the embodiments disclosed herein.

What is claimed is:
 1. A structure for two-phase cooling of integratedcircuits (IC's) comprising: an enclosure; a first semiconductorstructure; a second semiconductor structure located above the firstsemiconductor structure, wherein the first semiconductor structure andthe second semiconductor structure are within the enclosure; and anarrangement of metallic pillars located between the first semiconductorstructure and the second semiconductor structure, wherein thearrangement of metallic pillars electrically and thermally connect thefirst semiconductor structure to the second semiconductor structure, thearrangement of metallic pillars located in a fluid channel separatingthe first semiconductor structure and the second semiconductorstructure, wherein a fluid enters the enclosure at a fluid inlet andpasses between the first semiconductor structure and the secondsemiconductor structure through the fluid channel and exits theenclosure at a fluid outlet, the fluid cooling the arrangement ofmetallic pillars through a two-phase cooling process, and wherein thearrangement of metallic pillars are arranged in a staggered arrangementbetween adjacent fluid channel walls.
 2. The structure of claim 1,wherein the arrangement of metallic pillars provide structural supportbetween the first semiconductor structure and the second semiconductorstructure.
 3. The structure of claim 1, wherein the arrangement ofmetallic pillars are arranged in an inline arrangement between adjacentfluid channel walls.
 4. The structure of claim 1, wherein each metallicpillar in the arrangement of metallic pillars includes a first metallicpillar and a second metallic pillar joined by a reflowed solder.